Conferencing is well-known in the voice communication art. The simplest type of conference call is the one in which a third person joins an existing conversation. The greater the number of conferees or participants in a conference call, the more complex the circuitry. Circuits are known which accommodate the needs of hundreds of participants in a conference call.
In the ideal conferencing arrangement, any conferee can speak at any time, and any number of conferees can speak simultaneously with each being able to hear the combined voices of the others. In analog conference circuits, input signals from all conferees are summed to yield an output signal which is a composite of all the input signals.
In digital conference circuits, the summing of input signals from conferees will produce intelligible speech only if the digital code is linear, for example LPCM (linear pulse code modulated) or DPCM (delta pulse code modulated). Linear format digital signals can be summed because each bit has the same weight, i.e. represents the same stepsize or magnitude.
Some digital communications systems use a nonlinear compressed digital code format known as CVSD (continuously variable slope delta). One reason CVSD format is chosen over PCM format in certain applications is that CVSD allows a lower data rate since it is compressed, and consequently more channels may be multiplexed together at the same bit rate. For example, the PCM used in standard U.S. telephone systems typically requires a sample rate of 8,000 samples per second and 8 bits per sample, which equals 64 K bits per second per channel. Using CVSD, equal or better transmission quality is obtained at 32 K bits per second, and thus twice as many channels could be multiplexed on the same transmission medium.
A drawback of CVSD, however, is that direct summing of CVSD digital bit streams will not produce intelligible speech since the digital signals are based on a nonlinear compression conversion. During compression, different bits are assigned different weight and hence the bit positions are nonlinear. This nonlinearity enables compression but prevents straightforward summing of signals.
One system providing CVSD conferencing is disclosed in allowed U.S. patent application Ser. No. 960,491, "Digital Conferencing Apparatus" filed Nov. 13, 1978 by Donald L. Lambert, and assigned to the assignee of the present invention, now U.S. Pat. No. 4,215,247. Each of the conferees' CVSD signals are converted to linear signals by expansion from CVSD to DPCM representing the differentials of the voice signals. This expansion is the reverse of the above-mentioned nonlinear compression. Each of the expanded linear signals is summed and added to an element of the conference sequence which was previously stored in memory. This gives a linear digital signal indicative of the composite voice signal. The linear signal is then compressed to provide a CVSD digital sequence for transmission to the conferees.